As an individual ages, the eye is less able to accommodate, or bend the natural lens, to focus on objects that are relatively near to the observer. This condition is known as presbyopia. Similarly, for persons who have had their natural lens removed and an intraocular lens inserted as a replacement, the ability to accommodate is totally absent.
Among the methods used to correct for the eye's failure to accommodate is the use of the mono-vision contact lenses in which a person is fitted with one contact lens for distance vision and one lens for near vision. An alternative system provides both near and distance vision correction on each contact lens worn by the individual. As still another alternative, contact lenses in which at least one surface provides progressive power from far to near, or near to far, power are known.
The conventional contact lens product typically is produced using 1 to 3 base curves for the entire range of SKUs. This is disadvantageous in that the mismatch between the corneal curvature and the contact lens curve results in flexing of the lens on eye providing not only a poorly fitting lens, but also a lens in which the power on-eye experienced by the wearer is changed from the lens' prescribed power. Additionally, a tear film may form between the back surface of the lens and the corneal surface, which film is non-uniform in thickness and can cause additional power differences between prescribed and experienced power.
As an alternative, some contact lenses are designed using corneal topography in which lenses some or all of the back surface of the lens conforms to the shape of the corneal surface which it overlays. These lenses too are disadvantageous because of the expense entailed in providing lenses customized to an individual's corneal topography.